CS grad student here. Some mistakes I made were

• Not documenting code.
• Naming figures/datafiles poorly, so that you have no idea what they are in two weeks. It's best to have an automated way of labeling files if you'll be creating a lot of them.
• Storing data in an inefficient way (very bad if you're generating large amounts of data).
• Not using version control.
• Diving right into implementing an algorithm without first thinking about whether that's the best way to solve the problem.
• Being intimidated by tasks that looked difficult (they were rarely as hard as I thought they would be).